1. Field of the Invention
The present invention relates to a mist spraying apparatus and method, and an image forming apparatus, and more particularly to an apparatus and method for spraying a liquid in the form of a mist, by using ultrasonic waves, and to an image forming apparatus which records images by means of a group of ink micro-particles (ink mist) sprayed as a mist.
2. Description of the Related Art
There is an ink mist type of image recording apparatus (ink mist printer) which records images by generating a flow of ink mist (very fine ink particles) by means of ultrasonic vibration, and depositing this ink mist onto a recording medium as a group (cluster), (see, for example, Japanese Patent Application Publication Nos. 62-85948, 62-111757, 2-134250 and 5-57891). Furthermore, a composition of a print head using concentrated ultrasonic waves and nozzles has also been proposed (“Study on Ink Ejection of Print Head Using Focused Ultrasonic Wave and Nozzle” (Shumpei Kameyama, Hiroshi Fukumoto, and Shusou Wadaka, Journal of the Acoustical Society of Japan, Vol. 60, No. 2, (2004), pp. 53-60)), and “Multi-Level Ink Jet Printing with Mist Jet Ejected by Focused Ultrasonic Waves”, Hiroshi Fukumoto, et. al., Japan Hardcopy '99 Collected Essays).
Since the mist is a collection (cluster) of fine liquid droplets, it is highly subject to variations caused by prolongation in the flight time of the liquid droplets due to the effects of air resistance, and disturbance by external air flows. Therefore, a device is used to accelerate the movement of the fine liquid droplets forming the mist cluster by charging the droplets and applying an electric field (see Japanese Patent Application Publication Nos. 62-85948 and 62-111757, for example), but in this case, the mist cluster expands due to the occurrence of a Coulomb repulsion effect between the charged fine liquid droplets, and therefore it is essentially impossible to avoid enlargement of the dot diameter when the ink lands on the medium. In mist systems in the related art, there are many compositions which use the same electrode pair for charging and accelerating the liquid droplets, but in compositions of this kind, if the applied voltage is raised in order to shorten the flight time by applying an acceleration energy, then the amount of charge on the liquid droplets is also increased proportionately, and hence the dot diameter becomes larger due to increased Coulomb repulsion between the fine liquid droplets. This results in the deposition performance actually becoming worse.